U.S. Pat. No 4,304,584 describes the production of glasses exhibiting polarizing properties, i.e., glasses displaying dichroic ratios up to 40 and higher, from two types of silver-containing glasses: (1) phase separable glasses; and (2) glasses demonstrating photochromic behavior because of the presence of particles of a silver halide selected from the group of AgCl, AgBr, and AgI. The method for preparing the polarizing glasses contemplates two fundamental steps: (a) elongating the base glass articles under stress via such methods as drawing, extruding, redrawing, rolling, or stretching at temperatures between the annealing point and softening point of the glasses to cause the glass phases in the phase separable glasses or the silver halide particles in the photochromic glasses to become elongated and aligned in the direction of the stress; and (b) heat treating the elongated glass articles in a reducing environment at a temperature below the annealing point of the glasses, but above about 300.degree. C., to reduce at least a portion of the silver ions in the glass to metallic silver which is deposited in at least one of the elongated glass phases and/or along the phase boundaries of the elongated glass phases and/or deposited upon the elongated silver halide particles. The most efficient heat treatment is stated to comprise a temperature between about 375.degree.-450.degree. C. in a hydrogen atmosphere. Polarization was discerned in the visible and near infrared portions of the radiation spectrum.
In the case of phase separable, non-photochromic glasses, the operable compositions are asserted to reside in the alkali metal aluminoborosilicate field. Thus, suitable glasses will commonly consist essentially, in weight percent, of about 5-12% alkali metal oxide, 20-35% B.sub.2 O.sub.3, 1-15% Al hd 2O.sub.3, and the remainder SiO.sub.2, but with the proviso that, where Al.sub.2 O.sub.3 is included in amounts in excess of 5%, a specific phase separation agent such as CdO and/or F must be present at a level of at least 1%.
The base composition of an operable photochromic glass is noted as being essentially immaterial so long as AgCl and/or AgBr and/or AgI constitute the photochromic agents, although large amounts, viz., &gt;10% by weight total, of PbO and/or other readily-reducible metal oxides will be avoided.
That patent also discloses the production of composite bodies formed via concurrent extrusion of different glass compositions, that practice being operable with both phase separable glasses and photochromic glasses. In general, the composite body will consist of a thin surface layer or skin enveloping a thicker interior portion or core.
Thus, with respect to silver halide-containing photochromic glasses, it had been recognized in the art that the subjection of such glasses to high temperatures led to the growth of relatively large silver halide particles, the dimensions of the particles becoming so large as to cause light scattering with the consequent development of a hazy appearance. It was found, however, that the larger particles required less mechanical stress to effect elongation thereof and resisted the tendency to respheroidize to a much greater extent. Accordingly, it was deemed useful to form a laminated article comprising a thin skin glass which has been subjected to a relatively high heat treatment to generate large silver halide crystals therein and an interior portion that has been subjected to a less severe heat treatment to produce a transparent photochromic glass. Subsequent elongation of the composite body results in a thin skin exhibiting high polarization and a transparent core displaying good photochromic behavior. And, because the cross section of the skin glass is very thin, any haze developed therein will customarily have very little effect upon the optical transmission of the composite.
As can be appreciated, inasmuch as the polarization derived from the heat treatment conducted under reducing conditions is normally limited to a thin surface layer, typically 10-100 microns, the phase separable or photochromic glass need only comprise the surface layer of the composite, if so desired. Hence, the use of composite articles lends even wider versatility to the process. For example, only the skin layer is required to be elongated uniformly; the size, shape, and alignment of the particles in the core glass have little effect upon the polarization character of the final product. Furthermore, the production of laminated articles is economically attractive inasmuch as only the thin surface layer need include the relatively expensive silver-containing materials.
That patented procedure was shown to produce excellent polarization in the visible portion of the radiation spectrum with values therefor being reported at 550 nm and 600 nm, those wavelengths lying near the center of the visible region of the spectrum. For certain applications, however, it would be desirable to have glasses that exhibit good polarizing character in the infrared area of the spectrum, i.e., at wavelengths longer than 700 nm. The currently-available sheet-type and wire grid polarizers often display poor performances over the wavelengths between 700 nm-3000 nm because of their poor polarizing properties and/or poor chemical, mechanical, or thermal durability.
Therefore, the principal objective of the instant invention is to produce glasses demonstrating excellent polarizing properties over the infrared portion of the radiation spectrum, most preferably the region of 700-3000 nm. Polarizing properties up to the long wavelength transmission cutoff of the base glass, i.e., in the region of 3-5 microns, are, however, within the scope of the invention.
Another objective is to provide glass compositions especially suitable for exhibiting polarizing behavior of high quality over the infrared portion of the radiation spectrum, most preferably the region of wavelengths between 700-3000 nm.